In recent years, our lab has intensely studied photonic crystal microring resonators, where a grating inscribed on the inner sidewall of the microring couples and frequency splits otherwise degenerate counterpropagating modes of the microring. We have studied such resonators in the limit of strong gratings with large frequency splitting, with different unit cell geometries, in a regime where the grating doesn't backscatter light but instead couples it into free-space orbital angular momentum modes, and very recently, in a regime where dozens of modes are frequency split through a mode-by-mode, Fourier synthesis approach. In a recent paper, we have further advanced our understanding and engineering of such systems, considering the comparatively simple case where a single period grating is spatially displaced with respect to the center of the microring resonator.
With no spatial displacement, the grating couples and splits a single pair of counterpropagating microring modes. The spatial displacement, however, means that the orthogonality of other modes with respect to the grating is lost, and multiple sets of counterpropagating modes are coupled and split. We then show how this multi-mode splitting can be used to enable microresonator optical parametric oscillation in a microring resonator that would otherwise not satisfy the frequency matching conditions for this process. In comparison to our earlier work using multi-period gratings for the same purpose, this 'shifted grating' approach has a strong benefit in fabrication simplicity. Moreover, it can also be combined with some of the aforementioned approaches, including the Fourier synthesis technique.
This paper is part of a Feature Issue on Optical Microresonators.
Ref: Multi-mode microcavity frequency engineering through a shifted grating in a photonic crystal ring, X. Lu, Y. Sun, A. Chanana, U. Javid, M. Davanco, and K. Srinivasan, Photonics Research, 11, A72, (2023)